You can buy them at a grocery store rabbit hole (or in the buy catalog under appliances/misc)
and eat/chew them to get a nerfed minty breath buff (the one you get for brushing teeth in a sink).
It features a (lame) new mesh for the gum and a (even lamer) custom chewing animation for children and adults.

Gums are very small so pay attention where you place them.
If they are in a inventory they appear like a pack.
You can drag and drop them from a sim to another sim.
Sims will eat/chew gums autonomously (but keep in mind that they see them as food).
Simbots can't eat gums.